A thermal transfer process, an electrophotographic process, an ink jet; process and the like have been investigated energetically as techniques relating to color hard copies. A thermal transfer process has many advantageous merits as compared with other processes, since the maintenance and operation of the apparatus for the process are easy and the apparatus as well as corresponding supplies are low-priced.
Such a thermal transfer process includes a system where a thermal transfer dye donating material composed of a base film and a hot-melt ink layer formed thereon is heated with a thermal head so as to melt the ink. The molten ink is transferred to a thermal transfer image-receiving material to thereby form an image thereon. A system where a thermal transfer dye donating material composed of a base film and a thermotransferring dye-containing dye donating layer formed thereon is heated with a thermal head so as to transfer the dye under heat to a thermal transfer image-receiving material to form an image.
The latter thermotransfer system is especially advantageous for the recording of full-color images with high quality, since the amount of the dye to be transferred may easily be controlled by varying the amount of the energy to be applied to the thermal head so that recording of images with gradation is possible. However, as thermotransferring dyes usable in the system are much limited, and few dyes which satisfy all the necessary requirements are known.
Therefore, the necessary requirements for the dyes usable in the system are, for example, a spectral characteristic favorable to color reproduction, easy transfer under heat, resistance to light and heat, resistance to various chemicals, a property such that the sharpness of the dye hardly lowers after it has been transferred under heat to an image-receiving material, and freedom from re-transfer of the once transferred image. Further, a thermal transfer dye donating material containing the dye is required to be produced easily. Accordingly, development of cyan dyes capable of satisfying such requirements is desired.
Various thermal transfer cyan dyes have heretofore been proposed, and indoaniline dyes are considered comprehensively favorable above all. In particular, indoaniline dyes described in JP-A-61-31292, 61-35994 and 61-49893 are known to have excellent properties. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, even these dyes have the serious defect that the transferred images to be obtained still have a poor light-fastness.